In a constant current driving circuit for light emitting diodes (LED) with multiple outputs, each load branch includes one or more LEDs. When the voltages of two load branches are not the same, it is needed to balance the current of the two load branches, i.e. it is needed to realize that the total current output by the constant current driving circuit is assigned to each of the LED load branches as required.
Reference is made to FIG. 1 which is a circuit diagram of a conventional constant current driving circuit for light emitting diodes. A transformer Ta10 includes a secondary winding WT10 which is connected to two rectification loops to supply power to two load branches A1 and A2. In this circuit, the balance between the two load branches A1 and A2 is achieved by a current sharing transformer T10. If there are more than two loads, for example N load branches, N−1 current sharing transformers are needed. This circuit has the following disadvantage: in the case that one load branch connected to the current sharing transformer T10 is open circuited, to ensure normal operation of the other load branch connected to the current sharing transformer T10, there may be an over voltage occurred in the open circuited load, leading to damage of the circuit.
Reference is made to FIG. 2 which is a circuit diagram of an open-circuit protection circuit of a constant current driving circuit for light emitting diodes in the prior art. On the basis of the circuit shown in FIG. 1, in the circuit shown in FIG. 2, each of the loads in the constant current driving circuit for light emitting diodes is connected in parallel with an open-circuit protection circuit at the output of the load. The open-circuit protection circuit includes a zener diode ZD10, a first resistor R10, a second resistor R20, a first filter capacitor Cp10 and a thyristor SCR10. The zener diode ZD10, the first resistor R10 and the second resistor R20 are connected in series with each other and then connected in parallel with the load. The first filter capacitor Cp10 is connected in parallel with the second resistor R20. The thyristor SCR10 is connected in parallel with the load, and the gate electrode of the thyristor SCR10 is connected to the first resistor R10 at a terminal which is connected to the second resistor R20. When a load is open circuited or there is an over voltage, the zener diode ZD10 is in reverse conduction if the output voltage of the load branch is not less than the clamping voltage of the zener diode ZD10. After the current limiting by the first resistor R10 and the filtering by the first filter capacitor Cp10 and the second resistor R20, the thyristor SCR10 will obtain a gate electrode current. If this gate electrode current is not less than the threshold of the thyristor SCR10, the thyristor SCR10 is turned on and the load current flows through the thyristor SCR10, so that the voltage is decreased, thereby the LEDs in the other load branchs operate properly.
However, the above mentioned open-circuit protection circuit in the prior art has the following disadvantage: as shown in FIG. 2, when the thyristor SCR10 is turned on, the output capacitor Co10 or Co20 are shorted. Devices including SCR10 withstand larger current stresses because of the discharge of the output capacitor Co10 or Co20. Therefore devices which can withstand larger current are needed, so that the cost is increased. Taking two load branches as an example, in the case that the thyristor SCR10 in one load branch is turned on, the current sharing winding of the current sharing transformer T10 bears ½ of the output voltage. Since the current sharing transformer T10 bears a larger voltage, it is necessary to use a current sharing transformer with larger bulky.